1. Field of the Invention
The present invention relates to a sheet processing apparatus configured to perform folding processing of a sheet or a sheet bundle, and particularly to a sheet processing apparatus configured to perform folding processing to a sheet on which an image has been formed. The present invention relates to a sheet processing apparatus particularly without requiring complicated processing and capable of performing excellent folding processing even if a sheet bundle is thick or a large number of sheets is present.
2. Description of the Related Art
Conventionally, a sheet processing apparatus has been used which is equipped on an image forming apparatus, such as a copying machine or a laser beam printer, to perform processing such as binding after sheet are stacked and folding into a booklet form after sheets on which an image has been formed, which are discharged from an image forming apparatus main body, are successively received. For example, a sheet processing apparatus has been known which pushes the center of a sheet bundle by a pushing plate to fold the sheet bundle, presses the sheet bundle into the nip portion of a roller pair, and conveys and discharges the sheet bundle folded in two, thereby performing saddle stitch bookbinding processing.
In such a sheet processing apparatus, when the number of sheets is large, or a high-stiffness sheet is included in the sheet bundle, in a case where the sheet bundle is pushed by the pushing plate one time to be folded and is made to pass through the nip portion of the roller pair, the state of fold in the folded portion may not be sufficient. Consequently, a sheet processing apparatus is proposed in which a pushing plate or a folding roller pair performs a plurality of folding operations according to the number of sheets or the like. U.S. Pat. No. 5,876,027 discusses a configuration in which when the thickness of the sheet bundle or the number of sheets is equal to or less than a predetermined value, the sheet bundle is pushed only one time by a pushing plate and when it is larger than the predetermined value, the sheet bundle is pushed a plurality of times by the pushing plate.
However, the stiffness of a sheet bundle does not necessarily accurately correspond to the thickness of the sheet bundle or the number of sheets on one-to-one. Accordingly, in the above-described conventional sheet processing apparatus, the determined number of times of pushing may be excessive or insufficient.